The present invention generally relates to the art of electrochemical energy, and more particularly, to an electrode assembly, electrochemical cells in which the electrode assembly is used and a method for making the electrode assembly.
Batteries or electrochemical cells are typically volumetrically constrained systems which cannot exceed the available volume of the battery case. The size and resulting volume of the battery case are dictated by the space requirements available for the particular application. The components that make up a battery namely, the cathode electrode, the anode electrode, the separator, the current collectors, and the electrolyte all have to fit into the limited space defined by the battery case. Therefore, the arrangement of the components impacts on the volume of electrode active material that can fit into the case and the ease of manufacturing the unit.
Some typical electrode assemblies that attempt to maximize volumetric efficiency include the xe2x80x9cZxe2x80x9d folded electrode assembly which is disclosed in U.S. Pat. No. 3,663,721 to Blondel et al. In the xe2x80x9cZxe2x80x9d folded electrode, a unitary and continuous lithium anode is folded back and forth in a zig-zag fashion. The length of the individual folds determines the width of the electrode assembly. Individual cathode plates are positioned between pairs of the pleated anode electrode and electrically connected to one another. The design has some drawbacks including the requirement that separate cathode plates be inserted between each pair of adjacent layers of anode electrode, and the requirement that electrical connections be made between all of the inserted cathode plates. This arrangement increases the time and costs associated with manufacturing.
Another typical volumetrically efficient electrode assembly configuration is the xe2x80x9cjelly rollxe2x80x9d design in which the anode electrode, the cathode electrode, and the separator are overlaid with respect to each other and wound into a coil. Such a wound electrode configuration is desirable because the continuous anode and cathode electrodes require a minimal number of mechanical connections to their respective terminal leads, and the jelly roll type assembly is generally recognized as preferred for high discharge and current pulse applications. Use of the winding method, however, often limits the density of the electrodes because as an electrode is pressed more densely to its current collector it becomes more brittle and has a greater tendency to crack and flake off the screen especially when wound about a small radius bend. Also, the electrode material may delaminate along the length of the electrode causing the material to lose contact with the current collector screen.
Because stacked or flat folded cathode plates as described above do not create the stresses in the bend regions that are associated with winding, the plate method has been able to accommodate higher density electrodes and therefore has traditionally provided a cell stack of higher total electrode weight and capacity than is possible using the wound method.
What is needed is an improved wound cell stack with a relatively high density electrode for use in a prismatic (cuboid-shaped) or other non-cylindrical case.
The present invention meets the above-described need by providing a wound electrode having some regions where the electrode material is pressed to a high density and having some regions where the active material is pressed to a lower density.
The wound electrode has some regions that lie in the xe2x80x9cflatxe2x80x9d and some regions that are curved. Moving from the inside of the wound stack to the outside of the stack, the bend regions have increasing radii. The flat regions and the bend regions with a longer radius curve are pressed to a high density similar to a cell stack formed from the plate method. In bend regions making relatively xe2x80x9ctightxe2x80x9d or short radius turns, the active material is pressed to a lower density to accommodate the stresses associated with the bending during the formation of the cell stack.
In a preferred embodiment, a wound electrode cell stack has electrode material pressed to a lower density in the regions corresponding to the shorter radius turns. The tool that presses the active material to the current collector is shaped such that alternating high density and low density regions are located on both sides of the current collector screen.
The present invention also includes a method of manufacturing a wound cell stack as described above. The method includes the steps of simultaneously pressing the electrode active material onto the current collector screen to a high density in some regions and to a low density in other regions. The location of the low density regions is predetermined such that when the anode and cathode electrodes are wound into a cell stack the low density regions correspond to the regions where the shortest radius curves are formed in the wound cell stack.
To assemble the cell stack, the cathode electrode is placed in alignment with the anode electrodes and the combination is then wound as described below.